Field
Aspects of the present disclosure relate generally to ultrasonic imaging apparatuses, and more particularly, to a pixel receiver with capacitance cancellation for use in an ultrasonic imaging apparatus.
Background
An ultrasonic imaging apparatus may be used for obtaining electronic or digital images of certain items, such as fingerprints. An ultrasonic imaging apparatus typically comprises an ultrasonic wave transmitter including a transmitting piezoelectric layer (e.g., such as a polyvinylidene fluoride (PVDF) layer), a receiving piezoelectric layer (e.g., a PVDF layer), and a pixel receiver comprising a two-dimensional array of pixel sensors. Generally, the ultrasonic wave transmitter is situated below the pixel receiver, and the pixel receiver is situated below the receiving piezoelectric layer. The item-to-be-imaged, such as a user's fingerprint, is positioned above the receiving piezoelectric layer.
In operation, the transmitting piezoelectric layer of the ultrasonic wave transmitter is excited to generate an ultrasonic wave (e.g., a 10 MHz ultrasonic wave) upwards through the pixel receiver and the receiving piezoelectric layer until the wave encounters the item-to-be-imaged, such as a user's fingerprint. The ultrasonic wave reflects off the fingerprint and propagates downward towards the pixel receiver. The receiving piezoelectric layer converts the reflected wave into voltages at respective inputs of the pixel sensors of the pixel receiver. The voltages generated at the inputs of the pixel sensors are a function of whether the corresponding wave encountered a valley or ridge of the user's fingerprint.
The pixel sensors process the respective high frequency voltages to generate DC output pixel voltages. An analog-to-digital converter is provided to digitize the DC output pixel voltages. The digitized signals may then be processed by an image processor to perform various operations, such as fingerprint recognition, fingerprint database storage, and others.
In the past, the pixel receiver has been implemented using thin-film transistor (TFT) technology. However, circuits implemented using TFT technology generally have less voltage conversion efficiency and more noise than circuits implemented using other technology, such as complementary metal oxide semiconductor (CMOS) technology.